This invention concerns a mold, whether straight or curved, for the continuous casting of thin slabs. The invention can also be applied to molds for medium slabs.
The mold according to this invention is employed to produce thin slabs, and also advantageously medium slabs, suitable for subsequent rolling for the production of sheet or strip (coils).
The mold according to the invention has the purpose of producing slabs from 800 mm. to 3000 mm., or more, wide with thicknesses which may vary from 30 mm. to 90 mm. in the case of thin slabs and from 90 mm. to 150 mm. in the case of medium slabs.
Molds for the continuous casting of thin slabs are disclosed in the state of the art.
US-A-2,564,723 teaches the inclusion of a casting chamber in an intermediate position in the wide sides of the mold; this casting chamber has a surface conformed as a rhombus and not only enables a reserve of liquid metal to be formed which can thus feed the zone of the narrow sides but also enables the discharge nozzle of the tundish to be inserted so as to discharge liquid metal below the meniscus.
Next, it is necessary in the field of the rolling of sheet or strip that rolling campaigns should be carried out with different widths so as to meet market requirements.
US-A-4,134,441 therefore teaches the displacement of the narrow sides of the mold during the casting process so as to produce programmed widths of thin slabs.
SU-A-143.215 and JP-A-51-112730 disclose casting chambers with a curved peripheral development so as to prevent lengthwise cracks due to the sliding of the solidification skin, which has to take up substantial developments to arrive at the outlet section.
EP-C-149.734 includes the teachings of all these documents of the prior art and sets them forth in a coordinated manner so as to arrive at the same identical purposes.
All these documents of the prior art and also the present existing state of the art regarding thin slabs, namely slabs with a thickness of about an average value of 50 to 60 mm., provide for the casting chamber to extend vertically by about a quarter to a third, or by a maximum of a half, of the length of the mold. This condition, however, retains considerable problems of stress and strain on the skin while leaving the casting chamber and adapting itself to the surrounding walls.
So as to lessen these problems partly, very long and gently curved connecting portions have been provided in the zone of the change of direction, but these proposed embodiments do not obviate the existence of great metallurgical problems which reduce the withdrawal speed and the quality of the resulting product owing to lateral thrusts against the skin, the danger of removal of the skin and the turbulence caused by the modest dimensions of the casting chamber.
JP-A-51-112730, which concerns a mold to produce medium slabs for sheet and strip, provides for the casting chamber to be reduced progressively along practically the whole length of the mold so that the slab at the outlet of the mold has the desired nominal measurements with perfectly straight sides; but this proposal too, although favourable in itself, does not overcome all the problems of output and surface quality of the thin slab, for the quality is not always excellent with every type of steel thus cast. Moreover, the quality of the slab thus produced shows unacceptable quality defects sometimes during the rolling step.
DE-A-2.034.762 discloses a mold with a casting chamber with a through development and the pre-rolling of the enlargements produced in the slab leaving the mold so as to make the slab flat by the time it reaches the end of the discharge roller conveyor.
This document provides for through casting chambers with unchanging dimensions, but these chambers create problems of shrinkage and surface continuity of the skin.
WO-A-89/12516 offers two solutions substantially, of which the first, already contained in EP-A-230886, discloses a chamber with a rectangular plan and with its sides tapered to reach the normal section of the slab at an intermediate position in the length of the crystallizer; this solution in fact includes the same drawbacks, although partly reduced, as those contained also in the teaching of US-A-2,564,723.
The second solution provides for a through casting chamber having a constant width and a taper such that the sides at the center line of the casting chamber reach the dimensions of the slab outside the mold. This second solution includes a long and important pre-rolling process immediately downstream of the mold so as to reduce gradually the convex section. This second solution does not enable a smooth enough skin free of cracks to be produced nor, above all, the present necessary casting speeds to be reached.
Furthermore, this second solution makes difficult the alignment between the outlet of the crystallizer and the containing foot means. It also does not allow the start-up of the continuous casting.
Moreover, in the zone of maximum thermal stress for the slab, that is to say, in the zone of transition between cooling by conduction and cooling by convection, there is a component of thrust towards the center of the slab, and this component causes removal of the skin, combined bending and compressive stresses and deformations of the skin with the formation of hollows.